MRI, or Magnetic Resonance Imaging, (including: spectroscopy, conventional, and fast imaging techniques) is viewed as a conventional medical procedure having acceptable risks and certain concerns regarding bio-effects and patient safety. Of these concerns, electromagnetic energy adsorption may result in a host of undesired effects such as tissue or cellular damage. Absorption of electromagnetic energy by the tissue is described in terms of Specific Absorption Rate (SAR), which is expressed in watts/kg. SAR in MRI is a function of many variables including pulse sequence and coil parameters and the weight of the region exposed. In the United States, for example, the recommended SAR level for head imaging is 8 watts/kg.
T1ρ is commonly referred to as the longitudinal relaxation time constant in the rotating frame. T1ρ MRI produces images with contrast different from conventional T1- or T2-weighted images. T1ρ relaxation is obtained by spin-locking the magnetization in the transverse plane with the application of a low power radio frequency (RF) pulse(s). T1ρ relaxation is influenced by molecular processes that occur with a correlation time, τc, that is proportional to the frequency of the spin-lock pulse (γB1/2π). This frequency typically ranges from zero to a few kilohertz (kHz). In biological tissues, T1ρ is approximately T2, the spin-spin relaxation time constant, for very low amplitude spin-lock pulses and increases with higher intensity B1 fields. The sensitivity of T1ρ to low-frequency interactions facilitates the study of biological tissues in a manner that is unattainable by other MR methods. MRI using T1ρ-weighted contrast has been used to investigate and assess the condition of a variety of tissues such as breast, brain, and cartilage.
Contrast in magnetic resonance (MR) images derives from the magnetic relaxation properties of tissues. Variations in tissue relaxation times help to distinguish the healthy and the pathological states. An unconventional contrast mechanism called “T1ρ imaging” shows sensitivity to the breast cancers, early acute cerebral ischemia, knee cartilage degeneration during osteoarthritis, posttraumatic cartilage injury, and the intervertebral discs among people with nonspecific lower back pain. In addition, functional T1ρ imaging shows an augmented signal to brain activation and oxygen consumption (metabolism), and other applications.
Time constraints during an MR clinical examination place certain restrictions on T1ρ imaging sequences. For example, to diagnose a patient presenting chronic knee joint pain requires a pulse sequence with full volume coverage of the articular cartilage of the patella, femoral condyle and tibial plateau.
Present, pulse sequences are insufficient, however, for a standard clinical examination, because of either incomplete anatomical coverage, or prohibitively-long scan durations. That is, present, single-slice, 2D Turbo Spin Echo (TSE)-based acquisition schemes require an acquisition time on the order of a couple of minutes per slice. This time quickly increases if multiple slices are required. Compounding the time issue is the fact that multiple acquisitions are required to generate T1ρ maps of the tissue. Spin-locked Echo-Planar imaging (SLEPI) has a much briefer scanning time for single slice imaging, but the non-selective spin-lock pulse used does not allow for 3D data acquisition. A multi-slice 2D sequence with an equivalent adjacent slice-spacing to a 3D acquisition would result in cross-talk between slices due to imperfect excitation pulse slice profiles and thin slices are not achievable. Since T1ρ mapping involves collection of at least four 3D data sets at varying SL times, it is inherently inefficient.
Conventional 3D fast gradient-echo (FGRE) MRI, multi-slice and 2D EPI-based sequences typically require 20-25 minutes for gathering a single T1ρ map. Still further, conventional 3D T1ρ maps are typically collected with 2-4 mm slice thickness, since it is too time consuming to collect 3D maps with isotropic voxel sizes. T1ρ-weighted volume sets in clinical MRI studies examining pathologies in extended regions, such as, the articular surfaces of the knee joint, brain and heart, cannot be obtained under the time constraints of a viable clinical exam. Therefore, at least two views, e.g., sagittal and axial, are required to properly visualize anatomical structures in 3D T1ρ maps, which presently require a prohibitively long duration.